Electric drive vehicles including hybrid vehicles, electric cars, and other vehicles include a secondary battery (battery) mounted therein for storing electric energy for driving a vehicle. As the temperature of a secondary battery rises with charge and discharge due to an internal resistance of the secondary battery itself, the secondary battery must be cooled. Conventional techniques have therefore suggested providing a cooling fan around the secondary battery to forcibly cool the secondary battery.
JP 2010-158964 A, for example, discloses a cooling apparatus for vehicle electronic equipment, which includes a cooling fan near a secondary battery. JP 2010-158964 A discloses that a duct through which cooled air generated by driving a cooling fan flows bifurcates in the middle into two passages so that the route of the cooled air can be changed in accordance with the temperature of a charging device mounted on the vehicle. This cooling apparatus changes the command duty of the cooling fan in accordance with the temperature of a high-voltage secondary battery or an on-vehicle charging device which is to be cooled. This cooling apparatus further detects presence or absence of failure of the cooling fan, and if failure occurs, outputs failure information to a failure diagnosis function unit, and changes control for the electronic equipment in accordance with the content of the failure.
Detected parameters including the actual rotation speed of the cooling fan, for example, are often used for determining occurrence of abnormal condition of the cooling fan. It is desirable to determine presence or absence of abnormal condition as described above in a stationary state of the cooling fan where driving of the cooling fan is stabilized, because a significant change in the driving state (the rotation speed of a fan, and other parameters) of the cooling fan would cause an error in the detected parameters due to control delay and other defects, which makes it impossible to maintain the accuracy in the determination of presence or absence of failure.
In 2010-158964 A, the command duty is changed in accordance with the temperature of the high-voltage secondary battery or the charging device, which makes it difficult to place the cooling fan in a stationary state and therefore to accurately determine presence or absence of abnormal condition of the cooling fan. One possible solution to this problem would be to drive the cooling fan at a constant duty (constant rotation speed) without changing a command duty (command rotation speed) of the cooling fan. In this case, as the driving state of the cooling fan is stabilized, it is possible to accurately determine presence or absence of abnormal condition. However, continuous driving at a constant duty (constant rotation speed) irrespective of the state of the secondary battery and the driving state makes the rotation speed insufficient, causing insufficient cooling of the secondary battery and deterioration of the secondary battery, or conversely, increases the rotation speed to an unnecessarily high level, leading to undesirable noise and power consumption.